Accommodating bi-directional power flow in substation design

A consulting specifying engineer with a large commercial/industrial client that is planning to inject power into the grid must address aspects of that client’s substation design to accommodate the bi-directional flow of power.

The power source could be traditional such as thermally generated power or it could be renewable sources such as wind turbines and/or solar photovoltaics–the same issues apply. These distinct design considerations don’t exist for the simpler paradigm of a uni-directional power flow from the utility to the end-user.

The over-riding objective is to satisfy the utility’s need for situational awareness, a degree of visibility it needs to understand and manage how that bi-directional flow will affect its grid.

First, the consulting engineer needs to design for a greater degree of instrumentation in a substation that will handle a bi-directional flow of power. The design also will be influenced by the scale of power flowing onto the grid. If the bi-directional flow never exceeds 1 to 2 MW, situational awareness is achieved one way; if that flow is on the scale of, say, 100 to 200 MW, a different set of design considerations come into play.

The consulting specifying engineer’s first task, then, is to grasp the local utility’s requirements for situational awareness. Early in the substation design process, the consulting engineer needs to sit down with the local utility and understand what data points the latter requires. That could include bus voltages, power injection on the low side of the transformers–any number of data points might be required that, if not included in the initial design, unanticipated costs are likely later in the process.

The utility may even require a degree of control over the operation of the client’s substation in order to curtail power flowing onto the grid, or call for it, or even request volt/var support.

Both the sensors and the controls in this instance have implications for the substation’s communication network as well.

Further, the consulting engineer must not only understand the utility’s needs and requirements and communicate those to his/her client, but he/she may well serve as a mediator between the two parties. It’s best to stay ahead of the curve in order to properly perform design work and communicate and resolve issues affecting both parties.

Utility requirements may include specifying the equipment going into the substation, down to the level of vendor and device. For the situational awareness that a utility needs, it may require specific remote terminal units, or RTUs, and specific protocols to integrate with its SCADA and energy management system (EMS). At higher, transmission-level voltages, the utility may require specific protective relay equipment, again, down to the specific vendor make and model.

Of course, the utility also has safety concerns for its field crews when bi-directional power flows are integrated with the grid. Line crews may have to access the consulting engineer client’s substation to confirm that no power will flow while they’re working on a problem. Thus, utility-side needs will influence the physical design of the consulting engineer client’s substation, even the access controls.

So if you’re engaged by a client planning for or even just contemplating the future use of bi-directional power flows, be aware that a different set of rules apply. Engage with the affected utility at the earliest possible stage. The consulting engineer may find that the utility already has a set of design strategies, rules, and specifications in hand for just such an occasion. If the consulting engineer’s client is considering adding the capability for bi-directional power flow at some point in the future, certain design considerations might be made up-front at a prudent cost rather than incurring greater expense for a later retrofit.

If areas of uncertainty remain, where existing standards don’t apply to the situation, having a face-to-face meeting with the utility’s engineering staff is a must so that the areas in question can be worked out in a manner satisfactory for both sides.

The key is to understand the utility’s requirements and the client’s needs and future plans and ensure that both parties are well-informed and communicating in the early design stages for a successful project, while eliminating uncertainties.

Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation. He has more than 25 years of experience in the domestic and international electrical utility industries. Sciacca serves as the chair of two IEEE working groups that focus on cyber security for electric utilities: the Substations Working Group C1 (P1686) and the Power System Relay Committee Working Group H13 (PC37.240). Sciacca also is president of SCS Consulting.